1 Computer Networks LECTURE 21 Security Sandhya Dwarkadas Department of Computer Science University of Rochester Co 3-1 Chapter 8: Network Security Goals: understand principles of network security: • cryptography and its many uses beyond “confidentiality” • authentication • message integrity security in practice: • firewalls and intrusion detection systems • security in application, transport, network, link layers 8-2 Security An Example: Dyn and DDoS http://dyn.com/ddos / https://www.wired.com/2016/10/internet-outage- ddos-dns-dyn/ What is network security? confidentiality: only sender, intended receiver should “understand” message contents • sender encrypts message • receiver decrypts message authentication: sender, receiver want to confirm identity of each other message integrity: sender, receiver want to ensure message not altered (in transit, or afterwards) without detection access and availability: services must be accessible and available to users 8-4 Security
Transcript
1
Computer Networks
LECTURE 21
Security
Sandhya Dwarkadas
Department of Computer Science University of Rochester
Co
mp
uter
Net
wor
ks
(Tr
ans
port
Lay
er)
3-1
Chapter 8: Network Security
Goals: understand principles of network security:
• cryptography and its many uses beyond “confidentiality”• authentication
• message integrity
security in practice:• firewalls and intrusion detection systems
• security in application, transport, network, link layers
for each new plaintext symbol, use subsequent substitution pattern in cyclic pattern• dog: d from M1, o from M3, g from M4
Encryption key: n substitution ciphers, and cyclic pattern
• key need not be just n-bit pattern
8-13Security
Symmetric key crypto: DES
DES: Data Encryption Standard US encryption standard [NIST 1993]
56-bit symmetric key, 64-bit plaintext input
block cipher with cipher block chaining
how secure is DES?
• DES Challenge: 56-bit-key-encrypted phrase decrypted (brute force) in less than a day
• no known good analytic attack
making DES more secure:
• 3DES: encrypt 3 times with 3 different keys
8-14Security
Symmetric key crypto: DES
initial permutation
16 identical “rounds” of function application, each using different 48 bits of key
final permutation
DES operation
8-15Security
AES: Advanced Encryption Standard
symmetric-key NIST standard, replaced DES (Nov 2001)
processes data in 128 bit blocks
128, 192, or 256 bit keys
brute force decryption (try each key) taking 1 day on DES, takes 149 trillion years for AES
8-16Security
5
Public Key Cryptography
symmetric key crypto requires sender, receiver
know shared secret key
Q: how to agree on key in first place (particularly if never “met”)?
public key crypto
radically different
approach [Diffie-
Hellman76, RSA78]
sender, receiver do not
share secret key
public encryption key
known to all
private decryption key
known only to receiver
8-17Security
Public key cryptography
plaintext
message, m
ciphertextencryption
algorithmdecryption
algorithm
Bob’s public
key
plaintext
messageK (m)B
+
K B
+
Bob’s private
key K
B
-
m = K (K (m))B
+
B
-
8-18Security
Public key encryption algorithms
need K ( ) and K ( ) such thatB B. .
given public key K , it should be
impossible to compute private
key K B
B
requirements:
1
2
RSA: Rivest, Shamir, Adelson algorithm
+ -
K (K (m)) = m BB
- +
+
-
8-19Security
Prerequisite: modular arithmetic
x mod n = remainder of x when divide by n
facts:
[(a mod n) + (b mod n)] mod n = (a+b) mod n
[(a mod n) - (b mod n)] mod n = (a-b) mod n
[(a mod n) * (b mod n)] mod n = (a*b) mod n
thus
(a mod n)d mod n = ad mod n
example: x=14, n=10, d=2:(x mod n)d mod n = 42 mod 10 = 6xd = 142 = 196 xd mod 10 = 6
8-20Security
6
RSA: getting ready
message: just a bit pattern
bit pattern can be uniquely represented by an integer number
thus, encrypting a message is equivalent to encrypting a number
example: m= 10010001 . This message is uniquely represented by
the decimal number 145.
to encrypt m, we encrypt the corresponding number, which gives a new number (the ciphertext).
8-21Security
RSA: Creating public/private key pair
1. choose two large prime numbers p, q.
(e.g., 1024 bits each)
2. compute n = pq, z = (p-1)(q-1)
3. choose e (with e<n) that has no common factors
with z (e, z are “relatively prime”).
4. choose d such that ed-1 is exactly divisible by z.
(in other words: ed mod z = 1 ).
5. public key is (n,e). private key is (n,d).
KB
+K
B
-
8-22Security
RSA: encryption, decryption
0. given (n,e) and (n,d) as computed above
1. to encrypt message m (<n), compute
c = m mod ne
2. to decrypt received bit pattern, c, compute
m = c mod nd
m = (m mod n)e mod ndmagic
happens!c
8-23Security
RSA example:
Bob chooses p=5, q=7. Then n=35, z=24.
e=5 (so e, z relatively prime).
d=29 (so ed-1 exactly divisible by z).
bit pattern m me
c = m mod ne
0000l000 12 24832 17encrypt:
encrypting 8-bit messages.
c m = c mod nd
17 481968572106750915091411825223071697 12
cd
decrypt:
8-24Security
7
Why does RSA work?
must show that cd mod n = m where c = me mod n
fact: for any x and y: xy mod n = x(y mod z) mod n• where n= pq and z = (p-1)(q-1)
thus, cd mod n = (me mod n)d mod n
= med mod n
= m(ed mod z) mod n
= m1 mod n
= m
8-25Security
RSA: another important property
The following property will be very useful later:
K (K (m)) = m BB
- +K (K (m))
BB
+ -=
use public key first,
followed by
private key
use private key
first, followed by
public key
result is the same!
8-26Security
follows directly from modular arithmetic:
(me mod n)d mod n = med mod n
= mde mod n
= (md mod n)e mod n
K (K (m)) = m BB
- +K (K (m))
BB
+ -=Why ?
8-27Security
Why is RSA secure?
suppose you know Bob’s public key (n,e). How hard is it to determine d?
essentially need to find factors of n without knowing the two factors p and q
• fact: factoring a big number is hard
8-28Security
8
RSA in practice: session keys
exponentiation in RSA is computationally intensive
DES is at least 100 times faster than RSA
use public key crypto to establish secure connection, then establish second key –symmetric session key – for encrypting data
session key, KS
Bob and Alice use RSA to exchange a symmetric key KS
once both have KS, they use symmetric key cryptography
8-29Security
Chapter 8 roadmap
8.1 What is network security?
8.2 Principles of cryptography
8.3 Message integrity, authentication
8.4 Securing e-mail
8.5 Securing TCP connections: SSL
8.6 Network layer security: IPsec
8.7 Securing wireless LANs
8.8 Operational security: firewalls and IDS
8-30Security
Authentication
Goal: Bob wants Alice to “prove” her identity to him
Protocol ap1.0: Alice says “I am Alice”
Failure scenario??
“I am Alice”
8-31Security
in a network,
Bob can not “see” Alice,
so Trudy simply declares
herself to be Alice“I am Alice”
Authentication
Goal: Bob wants Alice to “prove” her identity to him
Protocol ap1.0: Alice says “I am Alice”
8-32Security
9
Authentication: another try
Protocol ap2.0: Alice says “I am Alice” in an IP packet
containing her source IP address
Failure scenario??
“I am Alice”Alice’s
IP address
8-33Security
Trudy can create
a packet
“spoofing”Alice’s address“I am Alice”
Alice’s
IP address
Authentication: another try
Protocol ap2.0: Alice says “I am Alice” in an IP packet
containing her source IP address
8-34Security
Protocol ap3.0: Alice says “I am Alice” and sends her
secret password to “prove” it.
Failure scenario??
“I’m Alice”Alice’s
IP addr
Alice’s
password
OKAlice’s
IP addr
Authentication: another try
8-35Security
playback attack: Trudy
records Alice’s packet
and later
plays it back to Bob
“I’m Alice”Alice’s
IP addr
Alice’s
password
OKAlice’s IP addr
“I’m Alice”Alice’s
IP addr
Alice’s
password
Protocol ap3.0: Alice says “I am Alice” and sends her
secret password to “prove” it.
Authentication: another try
8-36Security
10
Authentication: yet another try
Protocol ap3.1: Alice says “I am Alice” and sends her
encrypted secret password to “prove” it.
Failure scenario??
“I’m Alice”Alice’s
IP addr
encrypted
password
OKAlice’s
IP addr
8-37Security
record
and
playback
still works!
“I’m Alice”Alice’s
IP addr
encrypted
password
OKAlice’s
IP addr
“I’m Alice”Alice’s
IP addr
encrypted
password
Authentication: yet another try
Protocol ap3.1: Alice says “I am Alice” and sends her
encrypted secret password to “prove” it.
8-38Security
Goal: avoid playback attack
Failures, drawbacks?
nonce: number (R) used only once-in-a-lifetime
ap4.0: to prove Alice “live”, Bob sends Alice nonce, R. Alice
must return R, encrypted with shared secret key
“I am Alice”
R
K (R)A-B
Alice is live, and
only Alice knows
key to encrypt
nonce, so it must
be Alice!
Authentication: yet another try
8-39Security
Authentication: ap5.0
ap4.0 requires shared symmetric key
can we authenticate using public key techniques?
ap5.0: use nonce, public key cryptography
“I am Alice”
RBob computes
K (R)A
-
“send me your public key”
K A
+
(K (R)) = RA
-K A
+
and knows only Alice
could have the private
key, that encrypted R
such that
(K (R)) = RA
-K
A+
8-40Security
11
ap5.0: security holeman (or woman) in the middle attack: Trudy poses as Alice
(to Bob) and as Bob (to Alice)
I am Alice I am Alice
R
TK (R)
-
Send me your public key
TK
+A
K (R)-
Send me your public key
AK
+
TK (m)+
Tm = K (K (m))
+
T
-Trudy gets
sends m to Alice
encrypted with
Alice’s public key
AK (m)+
Am = K (K (m))
+
A
-
R
8-41Security
difficult to detect:
Bob receives everything that Alice sends, and vice versa. (e.g., so Bob, Alice can meet one week later and recall conversation!)
problem is that Trudy receives all messages as well!
ap5.0: security holeman (or woman) in the middle attack: Trudy poses as Alice (to
Bob) and as Bob (to Alice)
8-42Security
Chapter 8 roadmap
8.1 What is network security?
8.2 Principles of cryptography
8.3 Message integrity, authentication
8.4 Securing e-mail
8.5 Securing TCP connections: SSL
8.6 Network layer security: IPsec
8.7 Securing wireless LANs
8.8 Operational security: firewalls and IDS
8-43Security
Digital signatures
cryptographic technique analogous to hand-written signatures:
sender (Bob) digitally signs document, establishing he is document owner/creator.
verifiable, nonforgeable: recipient (Alice) can prove to someone that Bob, and no one else (including Alice), must have signed document
8-44Security
12
simple digital signature for message m: Bob signs m by encrypting with his private key KB,
creating “signed” message, KB(m)-
-
Dear Alice
Oh, how I have missed
you. I think of you all the
time! …(blah blah blah)
Bob
Bob’s message, m
Public key
encryption
algorithm
Bob’s private
key K
B
-
Bob’s message,
m, signed
(encrypted) with
his private key
m,K B
-(m)
Digital signatures
8-45Security
-
Alice thus verifies that:
Bob signed m
no one else signed m
Bob signed m and not m‘
non-repudiation:
Alice can take m, and signature KB(m) to court and prove that Bob signed m
-
Digital signatures suppose Alice receives msg m, with signature: m, KB(m)
Alice verifies m signed by Bob by applying Bob’s public key KB
to KB(m) then checks KB(KB(m) ) = m.
If KB(KB(m) ) = m, whoever signed m must have used Bob’s
private key.
-
--
+
+ +
8-46Security
Message digests
computationally expensive to public-key-encrypt long messages
goal: fixed-length, easy-to-compute digital “fingerprint” apply hash function H to
m, get fixed size message digest, H(m).
Hash function properties:
many-to-1
produces fixed-size msg digest (fingerprint)
given message digest x, computationally infeasible to find m such that x = H(m)
large
message
m
H: Hash
Function
H(m)
8-47Security
Internet checksum: poor crypto hash function
Internet checksum has some properties of hash function:
produces fixed length digest (16-bit sum) of message
is many-to-one
But given message with given hash value, it is easy to find another
message with same hash value:
I O U 1
0 0 . 9
9 B O B
49 4F 55 31
30 30 2E 39
39 42 D2 42
message ASCII format
B2 C1 D2 AC
I O U 9
0 0 . 1
9 B O B
49 4F 55 39
30 30 2E 31
39 42 D2 42
message ASCII format
B2 C1 D2 ACdifferent messages
but identical checksums!
8-48Security
13
large message
m
H: Hash
function H(m)
digital
signature
(encrypt)
Bob’s
private
key K B
-
+
Bob sends digitally signed
message:Alice verifies signature, integrity
of digitally signed message:
KB(H(m))-
encrypted
msg digest
KB(H(m))-
encrypted
msg digest
large message
m
H: Hash
function
H(m)
digital
signature
(decrypt)
H(m)
Bob’s
public
key K B
+
equal
?
Digital signature = signed message digest
8-49Security
Hash function algorithms
MD5 hash function widely used (RFC 1321) • computes 128-bit message digest in 4-step process.
• arbitrary 128-bit string x, appears difficult to construct msg m whose MD5 hash is equal to x
SHA-1 is also used• US standard [NIST, FIPS PUB 180-1]
• 160-bit message digest
8-50Security
Recall: ap5.0 security holeman (or woman) in the middle attack: Trudy poses as Alice
(to Bob) and as Bob (to Alice)
I am Alice I am Alice
R
TK (R)
-
Send me your public key
TK
+A
K (R)-
Send me your public key
AK
+
TK (m)+
Tm = K (K (m))
+
T
-Trudy gets
sends m to Alice
encrypted with
Alice’s public key
AK (m)+
Am = K (K (m))
+
A
-
R
8-51Security
Public-key certification
motivation: Trudy plays pizza prank on Bob• Trudy creates e-mail order:
Dear Pizza Store, Please deliver to me four pepperoni pizzas. Thank you, Bob
• Trudy signs order with her private key
• Trudy sends order to Pizza Store
• Trudy sends to Pizza Store her public key, but says it’s Bob’s public key
• Pizza Store verifies signature; then delivers four pepperoni pizzas to Bob
• Bob doesn’t even like pepperoni
8-52Security
14
Certification authorities
certification authority (CA): binds public key to particular entity, E.
E (person, router) registers its public key with CA.• E provides “proof of identity” to CA.
• CA creates certificate binding E to its public key.
• certificate containing E’s public key digitally signed by CA – CA says “this is E’s public key”
Bob’s
public
key K B
+
Bob’s
identifying
information
digital
signature
(encrypt)
CA
private
key K CA
-
K B
+
certificate for
Bob’s public key,
signed by CA
8-53Security
when Alice wants Bob’s public key:
• gets Bob’s certificate (Bob or elsewhere).
• apply CA’s public key to Bob’s certificate, get Bob’s public key
Bob’s
public
key K B
+
digital
signature
(decrypt)
CA
public
key K CA
+
K B
+
Certification authorities
8-54Security
Chapter 8 roadmap
8.1 What is network security?
8.2 Principles of cryptography
8.3 Message integrity, authentication
8.4 Securing e-mail
8.5 Securing TCP connections: SSL
8.6 Network layer security: IPsec
8.7 Securing wireless LANs
8.8 Operational security: firewalls and IDS
8-55Security
Secure e-mail
Alice:
generates random symmetric private key, KS
encrypts message with KS (for efficiency)
also encrypts KS with Bob’s public key
sends both KS(m) and KB(KS) to Bob
Alice wants to send confidential e-mail, m, to Bob.
KS( ).
KB( ).+
+ -
KS(m )
KB(KS )+
m
KS
KS
KB+
Internet
KS( ).
KB( ).-
KB-
KS
mKS(m )
KB(KS )+
8-56Security
15
Secure e-mail
Bob:
uses his private key to decrypt and recover KS
uses KS to decrypt KS(m) to recover m
Alice wants to send confidential e-mail, m, to Bob.
KS( ).
KB( ).+
+ -
KS(m )
KB(KS )+
m
KS
KS
KB+
Internet
KS( ).
KB( ).-
KB-
KS
mKS(m )
KB(KS )+
8-57Security
Secure e-mail (continued)
Alice wants to provide sender authentication message integrity
Alice digitally signs message
sends both message (in the clear) and digital signature
H( ). KA( ).-
+ -
H(m )KA(H(m))-
m
KA-
Internet
m
KA( ).+
KA+
KA(H(m))-
mH( ).
H(m )
compare
8-58Security
Secure e-mail (continued)
Alice wants to provide secrecy, sender authentication, message
integrity.
Alice uses three keys: her private key, Bob’s public key, newly
created symmetric key
H( ). KA( ).-
+
KA(H(m))-
m
KA
-
m
KS( ).
KB( ).+
+
KB(KS )+
KS
KB+
Internet
KS
8-59Security
Disclaimer
Parts of the lecture slides contain original work of James Kurose and Keith Ross. The slides are intended for the sole purpose of instruction of computer networks at the University of Rochester. All copyrighted materials belong to their original owner(s).
